Converting back deck to a Greenhouse

uny2bldApril 10, 2014

Hello everyone, I could use some advice.

My wife and I live in Upstate NY in grow zone 5a but we have pretty harsh winters. I want to convert my back deck into a greenhouse for Aquaponics. IÃ¯Â¿Â½d like some advice on building materials and design if possible.

The deck is 244" x 167" minus the 74" x 115" corner where the laundry room extends out from the rest of the house Ã¯Â¿Â½ but I might want to extend the greenhouse a little past the deck so my stock tubs can be near ground level giving me maximum space for racks above the line of stock tubs.

IÃ¯Â¿Â½d like to build off of the roof where the laundry room is which is 130" tall at the peak (which is near the center of the deck)

I am looking at 48 in. x 96 in. Multiwall Hurricane Panels (sold at Home Depot) for the walls and Tuftex 144-in x 26-in Clear Corrugated Polycarbonate Roof Panels (at LOWES) for the roof. I was also thinking about using pressure treated posts and 2 x 4Ã¯Â¿Â½s for the ceiling.

For the roof, would 12Ã¯Â¿Â½ long 2 x 4Ã¯Â¿Â½s be strong enough for our harsh winters in upstate NY? Also, what kind of spacing would I need for the ceiling? I know IÃ¯Â¿Â½d want as wide as possible for maximum lighting but I need it to be strong enough alsoÃ¯Â¿Â½ Advice?

How difficult would it be to make the greenhouse stay at 80 degrees all winter long?

80F all winter day and night?? That will be brutally expensive. You'd better do a lot of thinking before going that route.

Your design isn't energy efficient for that climate esp at 80F. You need full southerly exposure, insulated, double or triple wall, coverings on southerly facing sides and heavily insulated everywhere else. Heavily meaning just like you would insulate a house since you want to heat like a house.

Also at that temperature in your climate there will be major humidity issues for any walls attached to the house.

I think 80F is totally unrealistic. What are you wanting to grow that needs those temperatures? Your structure should be designed and build around what you want to grow and your climate.

The temperature requirement was more for the terrarium animals than the plants but it's not necessary...

I fear the roof will lose a lot of heat. Is there a better material I could use that woul let lots of sunlight in but increase the R value? Or - could I install 2 layers of polycarbanate roofing to have a pocket in between them?

I can insulate the walls heavily but I wanted some of the walls to be polycarbanate to let in more light / or is that a bad idea?

I'd do double or triple wall polycarbonate roof and those sides that will get a lot of winter sun. I'm not sure how that will compare to a single layer polycarbonate on each side of a 2x4. I'd think about double layer polycarbonate on each side. Or maybe triple wall acrylic just on top. I think the acrylic is even better. Try to talk to an expert at the supplier.

I'd be concerned that you don't have the snowload covered. It should be build as strong as the roof of a home unless steep enough to shed well.

And as Rich alluded to having a deck as a floor is an issue. That must be insulated and able to handle water passage. Plus you have no thermal mass that you'd ordinarily get from the soil.

Yes, the deck is raised. I plan on utilizing lots of drainage and I will definitely insulate underneath (I was thinking about using the spray foam insulation bc it's airtight and high r-value.) The deck is "L" shaped and 4 out of the 6 sides are surrounded by the house and I am building an insulated shed on the other side of the greenhouse that will be heavily insulated bc I'm using it as a water-purification and storage room) So, insulating it well should not be a problem. I will put raised beds on the other 2 sides so there will be no draft going under the deck whatsoever...

The roof is 12 feet long on the side that I'm really concerned about and it only drops 60 inches from one side of the other. I'm not sure if that peak is steep enough for 2x4 construction but I really didn't want to do 2x6's bc I felt they would stop too much light from getting through. Those concerns don't really matter if the roof would collapse from not being built strong enough though...

I like the idea of using double layers of polycarbonate. I'll look into acrylic but if it's the same acrylic material we use in the marine fishtank hobby, it doesn't hold up well with sunlight / UV and get's brittle and "crazes" making microfractures. Nonetheless, it's worth looking into if it is a different type.

You might want to search some of the Pot Grow set ups and see if there is something that can be adapted to your use. If they can grow it in closets, attics and garages....you might find that your garage is a better option

Good question. The stock tubs filled with fish will actually be at ground level sitting on a 2" slab of foam. I will be cutting holes in the deck to set these tubs into. Sorry, it seems I left a bunch of important details out of my original post.

Acrylic is expensive but is supposed to last longer than anything besides maybe glass.

Some info linked below on acrylic r and u values. There are similar tables you can google up about polycarbonate.

My guess is that single layer corrugated polycarbonate on each side of 2x4 roof would be about like double inflated poly. That r value can be found and isn't that high. Double layer polycarb on each side should have r near 4-5??

I personally wouldn't trust polycarbonate with the kind of snow load I imagine upstate ny gets -- even if your framing is strong enough it seems the panels themselves would collapse. Haven't tested or even looked it up...but worth checking out.

I'd suggest you build with a good slope so your snow slides off, and don't spray insulation under your deck...cuz of runoff.

I'd also suggest that you not build this on top of your deck unless you're willing to build it to the same standards you would for an extension to your home....which is a much different undertaking than even building a heated greenhouse to shelter a fairly elaborate aquaponics set up.

It seems to my (amateur, layman, lives in California and hasn't seen snow in years...) mind that you'd be much better off building this where you can have total sun exposure and you get some thermal help from the ground.

Either way...I've become a huge believer in using chain link fence top rail post to build my GHs with -- stronger than wood, and you don't have to make it a hoop house even though that's what you'll see online -- and you won't have to worry about it blocking your light.

Although acrylic has a much longer expected lifespan compared to polycarbonate, it is less flexible, more susceptible to hail damage and costs more than double. It also lets in more than twice as much UV light, which is not used by plants and is damaging to most everything else in the greenhouse. Also, the heat loss factors for comparable acrylic and polycarbonate panels are very similar.
One of the sources of this information is the attached link.